Tree: make checkStrictly responsive (#10867)

* Tree: fix model.TreeStore.checkStrictly doesn't change along with the change of tree.props.checkStrictly

* Update tree.vue

packages/tree/src/tree.vue

@@ -162,6 +162,10 @@
         Array.prototype.forEach.call(val, (checkbox) => {
           checkbox.setAttribute('tabindex', -1);
         });
+      },
+
+      checkStrictly(newVal) {
+        this.store.checkStrictly = newVal;
       }
     },